Small industrial and commercial boilers are widely used for heat and/or steam by universities, hospitals, commercial offices, apartments, food production plants, refineries and other industrial facilities. Estimates put the number of these boilers at over 160,000 units in the United States and the industry reports that several hundred new industrial and commercial boilers are installed each year. Unlike large utility boilers that often fire coal to produce high temperature steam to drive a turbine and generator for electric power production, these boilers produce low temperature and low-pressure steam and are generally fueled by natural gas or petroleum derived fuels, and in some cases fuel derived from biomass.
Regulations at the state and federal level are directed at reducing the emissions of nitrogen oxides from mobile and stationary sources including industrial and commercial boilers. In certain areas that have failed to meet attainment for ambient ozone or NOx standards these small boilers are required to reach emission levels as low as 9 parts per million (ppm) or even 5 ppm or less in the exhaust gas.
These low levels of emissions will require the use of ultra low NOx burners which are designed to tightly control the air, fuel and flue gas recirculation (FGR) rates. The result is to lower the oxygen content and temperature of combustion and reduce NOx emissions. High levels of FGR are required to achieve low NOx emissions and involve large fans capable of handling high volumes of hot flue gas. These fans consume large quantities of electric power to run their motors. The burners can be run more efficiently if allowed to operate at NOx levels above 15 ppm or even above 25 ppm and after treatment technology such as selective catalytic reduction (SCR) is used to reduce NOx emissions to levels below 10 ppm and even as low as 3 ppm or less.
One traditional after treatment approach to controlling NOx emissions involves the use of ammonia based SCR systems in which ammonia gas is introduced into the exhaust of a boiler upstream of a catalyst that chemically converts NOx to elemental nitrogen in the presence of ammonia. A difficulty with this approach is that the transport, handling and storage of ammonia often involves compliance with hazardous regulations. Due to the safety and health concerns, as well as the strict regulations, many small industrial and commercial institutions have restrictions on the presence of ammonia, making it unsuitable especially for applications such as hospitals, schools, food processors, office buildings and apartment buildings.
An alternative approach to the use of ammonia for SCR involves the use of urea solutions. Urea decomposes to byproducts including ammonia at temperatures above 400 F but the rate and completeness of the conversion to ammonia depends on factors such as temperature, residence time, injection technique, and droplet size. In aqueous solutions of urea the water must be vaporized and the urea decomposed and converted to ammonia prior to the catalyst. This requires greater design and operating care than the simple vaporization of aqueous ammonia.
However, use of the aqueous urea solution involves many disadvantages. For instance, urea is highly corrosive and attacks mechanical components of the SCR systems. Urea also tends to solidify upon prolonged exposure to high temperatures and the solidified urea will accumulate. Therefore, unconverted urea can foul reactor vessels, downstream ductwork, heat exchanger equipment and the SCR catalyst.
Furthermore, many industrial and commercial boilers have outlet gas temperatures only slightly above 500 F at a full load conditions and at lower loads the exhaust temperature can be below 400 F. This is generally too low for successful use of urea as a reagent. Additionally the residence time from the boiler outlet to a downstream SCR catalyst can be so short that the urea injected into the exhaust after the boiler outlet is not fully vaporized and decomposed to ammonia before reaching the catalyst.
While injection of urea into a higher temperature zone of a fire tube boiler has been demonstrated to provide conversion of urea to ammonia for SCR, as described in U.S. patent application Ser. No. 13/313,683 (Injector and Method for Reducing NOx Emissions from Boilers, IC Engines and Combustion Processes), the injection of urea directly into the furnace of a water tube boiler for SCR applications is not practical due to the tight tube spacing in the furnace convective zone which prevents adequate distribution of the reagent into the furnace gases. Urea deposition on boiler tube surfaces and corrosion of water wall surfaces in the boiler is also a concern with direct injection into a furnace.
There have been several attempts to overcome the disadvantages of known urea based NOx reduction systems. For example, U.S. Pat. No. 7,815,881 to Lin et al. describes the use of a flue gas bypass duct for injection of urea and for conversion to ammonia for SCR. U.S. Pat. No. 7,090,810 to Sun et al. describes the reduction of NOx from large-scale combustors by injecting urea into a side stream of gases with temperature sufficient for gasification and a residence time of 1-10 seconds.
However, the patents of Lin and Sun appear directed at large utility boilers. Utility boilers normally have sufficient heat input, flue gas temperatures and furnace residence times to generate 50 MW or more of electric power and are typically rated at 100 MW-800 MW or more. Whereas most industrial commercial boilers are rated below 300 million Btu/hour heat input, or roughly 30 MW equivalent.
Additionally, U.S. Pat. No. 5,296,206 to Cho et al. describes a process directed at large utility boilers, which achieves reagent flow rates up to 3,000 lbs/hr using a heat exchanger disposed in the flue gas pass such that a heated transfer medium is used to vaporize an aqueous reducing agent, which is preferably aqueous ammonia. However, Cho requires the use of a separate vaporizer vessel where the aqueous solution and heated air are mixed at the top of the vessel and the preferred outlet temperature is 250 F-500 F. The vaporization vessel of Cho represents an additional expensive piece of equipment that can be prone to plugging from the incomplete decomposition of urea, especially at the described low exit temperatures of 250-500 F described by Cho.
Due to their smaller size and generally lower baseline NOx emissions, the cost per ton of pollutant removed from an industrial boiler can be extremely high when control technologies such as those of Sun, Lin and Cho, which are designed for large utility boilers, are applied to small industrial and commercial boilers.
Other commercial processes for the conversion of urea to ammonia involve the use of supplemental heaters, burners or high temperature steam to provide heat for conversion of urea to ammonia and they often involve a separate storage vessel to hold the ammonia gas. U.S. Pat. No. 6,436,359 to Spencer and U.S. Pat. No. 6,322,762 to Cooper generally describe generating ammonia by heating urea under pressure. These systems can be complicated to control, require additional power to operate the heaters and are expensive relative to the cost of a small industrial or commercial boiler.
U.S. Pat. Nos. 5,968,464 and 6,203,770 to Peter-Hoblyn et al. describe the proposed conversion of urea to ammonia in the exhaust of a diesel engine by injecting urea onto the heated surfaces of a pyrolysis chamber mounted in the exhaust. The pyrolysis chamber is presented in the figures and described as a foraminous structure of sintered metal, glass or ceramic material inserted in the flue gas such that when urea is injected into the structure it is converted to ammonia which then exits the foraminous structure and mixes in the flue gas. However, this structure will quickly plug with unconverted urea byproducts. In U.S. Pat. No. 6,361,754 to Peter-Hoblyn et al. it is described to convert the urea solution to ammonia by injecting the urea into a heated line disposed within an exhaust pipe, with an optional heated vessel, and then releasing ammonia through a valve mechanism into the exhaust gases upstream of an SCR reactor. However, urea solution pumped into a small heated line would be prone to plugging of the line from urea decomposition products, which would present significant resistance to the continuing flow of urea solution through the line.
Therefore, what is needed is a simple and efficient method of converting small quantities of urea to ammonia without the need for secondary heaters and without the need for secondary storage of ammonia.